[0912.1460]
A dipole anisotropy of galaxy distribution: Does the CMB rest-frame exist in the local universe?

Authors:

Yousuke Itoh, Kazuhiro Yahata, Masahiro Takada

Abstract:

The peculiar motion of the Earth causes a dipole anisotropy modulation in the
distant galaxy distribution due to the aberration effect. However, the
amplitude and angular direction of the effect is not necessarily the same as
those of the cosmic microwave background (CMB) dipole anisotropy due to the
growth of cosmic structures. In other words exploring the aberration effect may
give us a clue to the horizon-scale physics perhaps related to the cosmic
acceleration. In this paper we develop a method to explore the dipole angular
modulation from the pixelized galaxy data on the sky properly taking into
account the covariances due to the shot noise and the intrinsic galaxy
clustering contamination as well as the partial sky coverage. We applied the
method to the galaxy catalogs constructed from the Sloan Digital Sky Survey
(SDSS) Data Release 6 data. After constructing the four galaxy catalogs that
are different in the ranges of magnitudes and photometric redshifts, we found
that the two samples of fainter magnitudes indicate a non-zero dipole
anisotropy with amplitudes greater than that of the CMB dipole by a factor 10
and in the angular direction consistent with the CMB direction, although the
dipole signal is weaker for the other two samples and is found sensitive to an
inclusion of the Southern Galactic Hemisphere region. The indicated bulk-flow
amplitude is also not inconsistent with the result implied from a stacked
analysis of the kinetic Sunyaev-Zel'dovich effect of X-ray luminous clusters in
Kashlinsky et al. (2008,2009). Finally we argue that an almost full-sky galaxy
survey such as LSST may allow for a significant detection of the aberration
effect of the CMB dipole having the precision of constraining the angular
direction to $\sim 20 $ degrees in radius.

This is something I always dreamt of doing... It is quite neat and the "detection" they have points at how difficult is to do this in practice... :)

My comments are:

a) Basically, it is quite clear that absolute photometric calibration is a big issue: that is probably the reason why the mean number density varies from the south to the north, etc. I don't think it is LSS contamination they are seeing, is just the non-uniform depth of the survey. Note that a single wrong patch spils over all ls in fourier space... It is also something that is most difficult to measure and correct for.

b) star-galaxy separation: there is this standard trick of marginalising over templates that has been used in CMB for ages and should work here as well: just add template covariance to you cov matrix with some large prefactor... Works like magic!

c) They claim that going to large zs helps because growth factor is lower and because galaxies are less evolving: but isn't them main effect simply from the fact that one is looking at larger scales and thus more uniform universe?

d) I also don't quite understand the hocus-pocus around contribution to diagonal elements from structure: they do some smoothing game. But instead, the eq. 16 with w_g(0) should give the right answer... Of course, you don't know what non-linear structure is doing exactly, but you can always parametrise this sensibly and then marginalise over it. Data should be able to constrain this internally (if all you want is dipole), I think.

I also think that LSST should be much better for this, not only because it has full sky coverage, but also, because of its scanning strategy it will revisit each patch of the sky many times, which should give much more uniform depth by simple averaging over atmospheric conditions...